1. Field of the Invention
This invention relates to a method for treating leachate from a sanitary landfill in order to remove contaminating constituents therefrom.
2. Description of the Related Art
Sanitary landfills contain a wide variety of wastes including organic wastes, some of which undergo anaerobic decomposition of organic material or garbage decay. When liquids such as rain water and ground water come in contact with these wastes, leachate is formed. Leachate generally contains high concentrations of various contaminants, including ammonia nitrogen compounds ("NH.sub.3 --N"), Chemical Oxygen Demand ("COD"), volatile organic compounds ("VOCs") and, possibly, heavy metals. Of these contaminants, disposal of untreated leachate having a high concentration of NH.sub.3 --N may present a serious problem due to its toxicity to fish populations. As a result, state and federal landfill regulations now contain stringent standards for collecting and treating leachate. Unfortunately, leachate is difficult to treat.
Basically, four technologies exist for the removal of the NH.sub.3 --N from leachate: ion exchange, breakpoint chlorination, biological removal and stripping towers. Unfortunately, each of these technologies has certain disadvantages. For example, ion exchange involves both substantial capital and operational costs due to both pre-treatment and removal equipment requirements. Although breakpoint chlorination requires minimal initial capital costs, this technology involves high operational costs, with the end products thereof often presenting discharge problems of its own.
Biological removal, which is predominantly used at present for removal of NH.sub.3 --N, requires both high capital and operational costs. In addition, this technology is sensitive to temperature, pH, and toxic metal concentrations, each of which randomly varies over a wide range in leachate obtained from landfills.
Stripping towers would appear to be particularly adaptable for removal of NH.sub.3 --N from landfill leachate as this method is capable of removing high concentrations of such compounds at the generally low flow-volumes of leachate. One of the aspects of stripping tower technology is to raise the pH of the leachate by adding an alkali, which largely converts ammonium ions, NH.sub.4.sup.+, to ammonia gas, NH.sub.3. The ammonia gas can then easily be stripped from the liquid leachate by diffusing air or another gas therethrough. This technology, however, has two major drawbacks: loss of efficiency during cold weather, and scaling caused by the addition of the alkali.
A landfill owner generally has three available options for treating leachate: (1) provide adequate on-site treatment of the leachate which meets the stringent government agency standards and directly discharge the treated leachate to surface waters; (2) provide proper pre-treatment of the leachate prior to discharge thereof into an off site wastewater treatment facility; or (3) discharge untreated leachate directly into an off site wastewater treatment facility which is capable of processing the untreated leachate. Conventional technologies used to treat high concentrations of NH.sub.3 --N and COD commonly include biological treatment plants, such as sequencing batch reactors or other aerobic biological processes in order to provide nitrification and COD reduction. Many of the publicly owned treatment facilities of smaller communities do not have the capability to provide the off site treatment facilities generally required for treating leachate.
It is known that an increase in temperature generally causes a corresponding increase in chemical activity. As a result, the rate of chemical processes utilized to remove contamination constituents from leachate can be enhanced by increasing the temperature of the leachate. Such an increase in temperature can be obtained by a variety of methods, including heating the leachate by combusting natural gas, which provides thermal energy of approximately 1,030 B.T.U./ft.sup.3. At most landfills, however, natural gas is not generally available, nor a free resource.
As an alternative, sanitary landfills which produce leachate also generally produce landfill gas, a substantial percentage of which is comprised of methane, an excellent low-cost heating source. Although the thermal energy available from landfill gas (approximately 400-600 B.T.U./ft..sup.3) is only about one-half of the thermal energy available from natural gas, the landfill gas is a free, and otherwise wasted, resource. Combustion of methane produces two important combustion products: (1) thermal energy, which can be used for heating purposes, and (2) carbon dioxide, which can be used to produce carbonic acid.
What is needed is a cost-effective method for treating leachate from a landfill, which utilizes the combustion products from a natural or landfill gas to not only accelerate the treatment process but also to reduce the pH of the treated leachate to a level whereby the leachate can be discharged directly into the environment.